1. Field of the Invention
The present invention relates to an automatic focussing adjusting device and, in particular, to an automatic focussing adjusting device which adjusts the automatic focussing of a taking lens of a camera by detecting the amount of de-focussing of an object.
2. Description of the Related Art
There is already known a system in which the amount of de-focussing of an object is detected and, in accordance with the detected de-focussing amount, the focussing of a taking lens of a camera is adjusted.
This system makes uses of the fact that a focussed pattern, formed by the light that has pressed through the upper half section of the taking lens, is out of phase with a focussed pattern formed by the light that has passed through the lower half section of the taking lens on a focal plane when they are out of focus. That is, the amount of out-of-phase is considered to be the amount of de-focussing (amount of out-of-focus) and the focussing is adjusted in accordance with the amount of de-focussing.
Incidentally, the amount of de-focussing of a zoom lens varies with the focal length of the zoom lens and the positional relation of a focus lens group. In FIG. 13, there is shown the structure of the zoom lens Z and, in this figure, A designates a focus lens group, B a varier lens group, C a compensator lens group, and D a master lens group. Here, the lens groups other than the focus lens group A in the zoom lens are referred to as a magnification varying lens group E.
In the above-mentioned structure, a first the focus lens group A is used to perform the focussing adjustment so as to obtain an image Q1. The magnification of the image Q1 is varied by the varier lens group B to provide an image Q2. Further, the movement of the image Q2 is corrected by the compensator lens group C and then after the image Q2 is focussed by the master lens group D on the focal planes of an image pickup device, a focus detection element and the like forms an object image Q3.
The construction of the zoom lens Z, as shown in FIG. 14, can be simplified by the focus lens group A and magnification varying lens group E. FIG. 14(1) shows an in-focus state, FIG. 14(2) shows a state in which the focus lens group A is situated forwardly by a distance .DELTA.X more than in-focus, a so called forwardly de-focussed state, and FIG. 14(3) shows a state in which the focus lens group A is situated rearwardly by the distance .DELTA.X more than in-focus, a so called rearwardly de-focussed state. In these figures, a represents "a" distance between the magnification varying lens group E and the image Q1, "b" a distance between the magnification varying lens group E and the image Q3, "fa" the focal length of the focus lens group A, "fe" the focal length of the magnification varying lens group E, and .DELTA.P1, .DELTA.P2 the amounts of de-focussing, respectively,
In FIGS. 14(2) and 14(3), in spite of the fact that the focus lens group A is moved forward or backward by the same amount of movement .DELTA.X from a reference position in which the focus lens group A is in focus, the amounts of de-focussing provide different values of .DELTA.P1, .DELTA.P2, respectively.
Here, if the focal distance of the zoom lens Z is expressed by "fz", the magnification of the magnification varying lens group E by "m", the focal distance by "fe", and the amount of de-focussing by .DELTA.P, then the amount of movement of the focus lens group A can be represented by the following equation: ##EQU1## (where, m=fz/fa.)
This is disclosed in detail in laid open Japanese Patent Application No. 58-217907 (Tokkai).
Conventionally, the above-mentioned optical properties of the zoom lens are taken into consideration and the above equation (1) is used to determine the amount of movement of the focus lens group A necessary for performing the focus adjustment.
When the zoom lenses are mass-produced, the individual zoom lenses vary in their magnification m and focal distance fe of the magnification varying lens group in the above equation (1) due to the variations of precision in machining and assembling. In fact, this provides an error in the amount of movement .DELTA.X.
In general, the focal distance of a lens, when actually produced produces an error of .+-.3% with respect to the design value thereof. Also, distances between the respective lens groups forming the zoom lens yield errors with respect to the design values thereof in assembling, which appear as an error in the focal distance of the whole zoom lens.
As mentioned above, even if the amount of de-focussing .DELTA.P is detected accurately, the above-mentioned factors cause an error in the amount of movement .DELTA.X of the focus lens group. Due to this, when the auto-focussing (or, Auto Focus which will be simply referred to as AF) of the zoom lens is controlled with the magnification m and focal distance fe having been previously set as fixed values, focussing can be achieved by a single AF operation but for the above-mentioned error, however, actually several AF operations are necessary for the focussing. That is, the AF control cannot be performed smoothly.
On the other hand, the amount of de-focussing .DELTA.P is calculated in accordance with the detected output of a focus detect element such as a TCL (Through Camera Lens) module or the like, as will be described in detail later. The TCL module is mainly composed of a group of light receiving elements which are arranged linearly at given intervals, and a group of microscopic lenses (fly eye lenses) which are disposed forwardly by a given distance from the light receiving element group.
In the TCL module, dimensions such as spacings between the adjoining light receiving elements in the light receiving element group, spacing between the light receiving element group and the microscopic lens group, spacings between the optical axes of the adjoining microscopic lenses in the microscopic lens group, and the like, produce manufacturing errors with respect to their design values due to their different machining and assembling precisions, when they are mass-produced.
These manufacturing errors have an effect on the detected output of the focus detect element and thus in the amount of de-focussing .DELTA.P that is calculated in accordance with the above detected output there is produced an error which is caused by the above-mentioned manufacturing errors in the focus detect element, which, in the end, produces an error in the amount of movement .DELTA.X of the focus lens group obtainable from the equation (1) and necessary for focussing.
This is not limited to the TCL module, but it can also apply similarly to other focus detect elements of a type that detects a focussed state on the focal plane of an object image by means of a group of sensors arranged one-dimensionally or two-dimensionally in a plane vertical to the optical axis in the neighborhood of the focal plane.
However, in the conventional zoom lens automatic focussing adjusting device, the automatic focussing control is performed without giving any consideration to the errors caused by the above-mentioned manufacturing error of the focus detect element and included in the amount of de-focussing .DELTA.P calculated in accordance with the detect output of the focus detect element. Therefore, if the error caused by the manufacturing error of the focus detect element and included in the amount of de-focussing .DELTA.P is large, then the error produced in the amount of movement .DELTA.X of the focus lens group calculated from the equation (1) is also large, so that the focussing cannot be achieved by a single AF operation and, therefore, the AF control cannot be performed smoothly.